Carrier amplitude-control system



Aug. 28, 1928.

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Fidel Fig 2 W5 16 INVENTOZS Qfifilachvell wk By Eli fel .4 TTORNE Y lg L I; l- I Patented Aug. 28, 1928.

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WOOD, NEW JERSEY,

ABSIGNOBS TO AMERICAN comm,- A CORPORATION 01' NEW YORK.

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' This invention relatesto a carrier or radio .system characterized by the simultaneous transmission of-the carrier frequency and one or both of its side bands and particularly to a method and means for controlling the amplitude of the carrier frequency in accordancewith variations in am litu e of the side band corresponding to c the instantaneous va uesoof the signal frequency load being transmitted over the carrier or 'radio system.

- In the method heretofore employed in radio transmission or broadcasting, and also in carrier systems in which the carrier frequency is simultaneously'transmitted with one or both of the side bands resulting from modulation of the carrier by a frequency or band of frequencies representin the load to be transmitted by the carrier, t e ampli tude of the carrier frequency r se remains substantiall constant, w ereas the amplitude of the si e bands. increases and decreases in accordance with the instantaneous Values of the frequency or frequencies em ployed .in modulating the carrier. For example, during the transmission of speech over a carrier system, there will be times when the side bands will cease to exist,

which times correspond to a ause in the speech being impressed upon t e transmit-- ter of the system. In such intervals, the carrier frequency continues to flow over the system to the detector at the receiving station. Any extraneous interference or noise lying Within the frequencies of the slde band will be demodulated with this carrier by the detector at thereceiving station. The object-ion to this situation is that during these pausesthe receiving system is as susceptible to noise conditions on the circuit as it is when the speech band is being received.

One of the objects of this invention is to overcome the objection set forth above by causing the sensitivity of the receiving system to be reduced during periods of light load, or no load, corresponding, for example, to pauses in a speech or musical program. This result is accomplished by causing the amplitude of the carrier to be varied directly with variations in the magnitude of the audio frequency input, and simultaneously subjecting the gain applied to the side nges in ,such

work 2 which bands to control, but in an inverse sense.

This-invention will be clearly understood from the followmg description when read in connection with the attached drawing, of which Figure 1 shows schematically a system characterized byl transmission of a carnet of va amp 'tude, and Fi 2 shows schematl ly a system in which t e amplitude of the carrier is varied directly with the amplitude of the voice input, and the gain applied to the side band varied in veisely with the amplitude of the voice inpu In 1 the line L represents a signal input circuit having connected with it suitab e apparatus for picking up the signals as speech or music to be transmltted over the carrier system. The line L is connected with the modulator M by the transformer 1, the connection including a net- I may be designated a delay OII'CllIl), the function of which is to retard the signal which is about to be impressed upon the modulator M and to prevent its e ect thereon until certain other operations have taken place. Bridged across the line L at the points 3 is a carrier amplitude control circuit comprising a rectifier 4, a low pass filter 5, a push-pull modulator M, havin a carrier frequency generator 6 connecte with the input side thereof. This carrier amplitude control circuit is connect- .ed with the l1ne L by means'of the transformer 7, and the output side of this control circuit is connected with the input circuit of the modulator M by the transformer 8, the connection also including a filter or tuned circuit 9 which transmits substantially only the carrier frequency generated by the source 6. The output side of the modulator M is connected by he transformer 10 with a carrier circuit 2 which may contain an amplifier and a filter such as 13, and may be connected with the main transmission line in any one of a number of well-known ways. Furthermore, a receiving circuit, which may be of any well-known type adapted to receive the carrier frequency and its side band, and to detect the audio-frequency signal, maymbe connected with the mam transmission e.

of the side bands for transmission and impressed at the points 3 across the primary.

of the transformer 7. The tube 4, which is arranged to function as a rectifier, rectifies the speech wave, and the resultant rectified current is smoothed out by means of the network 5 constitutinga low pass filter so that onl its major variations are transmitted. Thls rectified current as smoothed out by the low pass filter 5 flows through the resistances 11 and 12 in series across the grids of the push-pull modulator M The modulator grid is accordingly unbalanced, so that a wave of carrier frequency from the source 6 will be impressed upon the secondary winding of the transformer 8, the amplitude of which will depend upon the degree of unbalance of the said modulator. This in turn depends upon the magnitude of the rectified current flowing through the resistances 11 and 12 in series, which establishes a potential difference across them varying with variations in the magnitude of the signal current. The relation between the amplitude of the carrier and that of the voice input may or may not be arranged to be exactly linear, as desired. The filter 9 is designed to pass only slow variations of the carrier frequency, thus eliminating any residual side band components that might be produced by signal variations in the rectified speech frequency not eliminated by low pass filter 5. This carrier frequency will be modulated by the speech frequency imressed by the transformer 1 upon the moduator M and the resultant side bands and the carrier frequency will be impressed by the transformer 10 upon the circuit L. A filter 13 may be included to eliminate one an amplifier may also be inserted. The delay circuit 2, by retarding the speech signal, enables the change in the amplitude of the carrier current to be effected before the speech reaches the input of the modulator M,. It will accordingly be seen that as the speech diminishes in intensity, the amplitude of the carrier is likewise reduced, and by proper adjustment of the rectifier, entire suppression of the carrier in so far as it affects the input of the modulator M, may be effected when there is a pause or cessation of speech.

In the system before described, there may be distort-ion of the speech as detected at the receiving station due to the fact that the volume changes of the voice will be exaggerated. This follows because the modulator circuit and the demodulator circuit no longer func tion as substantially constant gain devices, since their gain depends on the supply of carrier current, which is variable and greatthat the effective grid potentials will be est when the voice input is greatest, and vice versa, least when the voice input is lowest. The arrangement shown in Fig. 2 serves to overcome this defect in the arrangement shown in Fig. 1 by feeding the side bands and the carrier frequency to the line throu 11 separate sources, and causing the ain of the modulator which produces the si e bands to fall off when the amplitude of the carrier supplied to the line is increased, and vice versa. I In Fi 2 of the drawing, the signal input circuit has bridged across it at the points 3 a control circuit having a rectifier 4 and a low pass filter 5, which elements while shown schematically in Rig. 2, are preferably of the form shown in greater detail in F' ..1. The output circuit of the rectifier is brid ged across the resistances 11 and 12, connected with the grids of the balanced modulator M and is also connected across a condenser 14 in the common or middle conductor of the modulator M,. The source of carrier frequency 6 is connected across the impedances 17 and 19 in the common conductors of the modulators M, and M The modulator M, is connected with the input circuit L, by the transformer 1, and is connected with an output eircuit by the transformer 10, which connection may include a filter 13 designed tov pass one of the side bands of modulation. The modulator M,, which controls the amplitude of the carrier current supplied to the line, is connected by a transformer 15 with the line, the connection including a filter 16 corresponding to filter 9 in Fig. 1. The carrier frequency current, which is supplied by the source 6 to the modulator M being applied across the impedance 17 in the common conductor of the modulator, is varied in amplitude in the same way in which the carrier current is varied in Flg. 1 by means of the rectified signal current flowing through the resistances 11 and 12. The gain applied to the side bands is controlled by a portion of the rectified signal current that flows over the branch circuit 18, which is bridged across the condenser 14 in the common conductor of the modulator M,. This rectified current, which, of course, varies with the level of the inn lois

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signal upon the line L,, produces a corre- 1'15 spondingly'varying potential to the grids of the modulator M,, and is so applied that it will reduce the gain ap lied to the side bands whenever the amplitu e of the carrier is increased, and, vice versa, it will increase the gain applied to the side bands, when the amplitude of the carrier diminishes. It is intended that for an increasing speech input current the connection shall be so arranged varied in such a manner as to decrease the gain of the modulator M, while for a decreasing speech input current, the gain of the balanced modulator M, will be increased.

The amplitude of the carrier may be varied he in accordance with any root or power of the voice in ut, and the gain applied to the side band wi lbe given an inverse variation. This will tend to insure at the receiving station the proper relation between side band and carrier amplitude to avoid the possibilit of distortion mentioned hereinbefore, an at the same time, a reduction is likewise effected in the carrier amplitude durin the period in which littleor no speech is being transmitted.

While this invention has been disclosed in certain forms, it is to ,be understood, of course, that such forms are merely schematic, and that the invention is capable of embodiment in other and different forms without departin from the spirit and scope of the ap nde claims.

at is claimed is:

1. In a carrier or radio signaling system in which a. carrier frequency is modulated b a signal frequency, the method which consists in producin a carrier frequency, varying the energy lever of the carrier frequency y variations in the level of the signal frequency and simultaneously modulatlng by the said s'gnal the carrier fre uency as thus varied.

2. In a carrier or ra io signaling system in which a carrier frequency is modulatedby a signal frequency, the method which consists in controlhn the amplitude of a carrier frequency by t e amplitude of a signal freuency, and simultaneously modulat'ng by t c said signal frequency the said carrier frequency as thus controlled 3. In a carrier or radio signaling system in which a carrier frequency is modulated by a signal frequency, the method which consists in producing a signal frequency and also a carrier frequency, rectifying a portion of the signal frequency, varying the level of the carrier frequency by and in accordance w1th changes in amplitude of the rectlfied signal frequency, and modulating the sand carrier frequency by the said signal frequency.

4. In a carrier or radio signaling system in which a carrier frequency is modulated by a signal frequency, the method of regulating the amplitude of the carrier frequency transmitted over the said s stem, which consists in rectifying a portlon of the signal f uency energy, controlling the amplitude $31.

e carrier frequency in accordance with changes in magnitude of the energy level of the signal frequency as represented by the rectified signal current, and modulatm the said carrier frequncy by the signal frequency.

5. In a carrier or radio signaling system in which a carrier frequency is modulated by a signal frequency, the method which consists in modulating a carrier frequency by a signal frequcncv, transmitting the frequencies resulting from modulation, and controlling the said modulation so that the amplification applied in the modulation process varies inversel with variations in the level of the signal requency, and simultaneously transmitting and controllin the said car'- rier frequency so that its love as transmitted varies directly with variations in the energy level of the said signal frequency.

6. In a carrier or radio signaling system in which a carrier fre uency is modulated by a signal frequency, t e combination with a source of carrier frequenc of a source of signal frequency, a modu ator connected w1th both sources to modulate the said carrier frequency by the said signal frequency, and means connectedwith the said source of signal frequency and-responsive to variations in the ener y level of' the signal frequency to control t e amplitude of the carrier frequenc I 7. n a carrier or radio signalin -s stem in which a. carrier frequency is modufated b a signal frequency, the combination wit 1 a source of carrier frequency of a source of signal frequency, a modulator connected with both sources to modulate the carrier-frequency by the signal frequency, means connected w..th the source of signal frequency to rectify a portion thereof, and means connected with the source of carrier frequency and controlled by the rectified signal voltage to vary the amplitude of the carrier frequency as transmitted in accordance with varlations in the energy level of the signal frequency.

8. In a carrier or radio signaling system in which a carrier fre uency is modulated by a signal frequency, t e combination with a source of carrier frequency of a source of signal frequency, a modulator connected with both sources to produce side bands of the said carrier frequency by the said s gnal frequency, a transmission circuit connected with the output side of said modulator, a second modulator of the balanced t e connected between the said source of uarrier frequency and the said transmission circuit in such manner that the transmission of the carrier frequency is normally suppressed, and rectifying means connected between the said source of signal frequency and the said second modulator to apply rectified signal voltage to the second modulator to effect the transmission of the carrier frequency by unbalancing the sa d modulator, the degree of unbalance varying directly with variag tions in the energy level of the said signal frequency.

9. In a carrier or radio signaling system in which a carrier frequency is modulated by speech frequency, the method which consists in controlling the amplitude of the carrier in accordance with variations in the envelope of the speech band, and varying the amplitude of the side band inversely with changes of the amplitude of the carner.

10. In a signaling system, a source of low 13, The'method frequency to correspond to changes iri'the amplitude of the low frequency signals, and means for superimposin the low frequency signals on the carrier fi 'equency controlled as to its amplitude in accordance with the low frequency signals.

11. In a s'gnaling system, a source of voice frequency signals, means for selecting from said voice frequency signals a low frequency wave corresponding to changes in the amphtude of the voice frequency signals a carrier frequency, means for controlling the amplitude of the carrier frequency by the amplitude of the selected low frequency wave, and means for modulating the controlled carrier frequency by the voice frequency signals for transmission to a distant point.

12. The method of signaling which consists in generating a carrier transmitting frequency, controlling the amplitude of said carrier transmitting frequency in accordance with the amplitude of incoming voice frequency signals, modulating said controlled carrier transmitting frequency by the incoming voice frequency signals, and transmitting the controlled carrier transmitting frequency after being modulated by said voice frequency signals.

sists :in generating voice frequency signals, and superimposing said voice frequency slgnals on a carrier frequency controlled as to its amplitude by the amplitude of the said voice frequency signals.

14. The method of controlling the amplitude of the carrier frequency in a signaling system which consists in generating voice frequency signals, selecting from the voice frequency signals a wave corresponding to the envelope of the voice fre uency signals, controlling the amplitude of t e carrier frequency by the envelope of said voice frequency signals, and modulating the carrier frequency controlled as to its amplitude by the voice frequency signals.

15. In a signaling system, the combination with a source of low frequency signals, of a source of carrier frequency, means for controlling the amplitude of the carrier frequency in accordance with the envelope of the low frequency signals, and means for modulating the controlled carrier frequency by the low frequency signals for transmission to a distant point.

16. In a system for the transmission of signals, the combination with a source of low frequency signals, of means for selecting from the low frequency signals a wave corresponding to changes in the amplitude of the low frequency signals, a source of carrier frequency, means for modifying the amplitude of the carrier frequency by the of signaling which conwave selected from the low frequency signals, and means for superimposing the low frequency signals on the modified carrier frequency fortransmission to a recelving means for superimposin the low frequency signals on the modifie carrier frequency for transmission to a distant station.

18. In a system in which a carrier frequencyis modulated by low frequency signals, means for controlling the amplitude of the carrier frequency in accordance with the amplitude of the low frequency signals, said means comprising afilter for selecting from the low frequency signals a frequency corresponding to the envelope of the low frequency signals, a rectifier for rectifying the selected frequency, and a modulator upon which the carrier frequency and the rectified selected frequenc are impressed for changing the amplitu e of the carrier frequency.

19. In a signaling system, the combination with asource of low frequency signaling oscillations of varying amplitude, of a source of high frequency oscillations, a filter .for selecting from the low frequency signaling oscillations a wave representing the variation in the amplitude of the low frequency signaling oscillations, means for controlling the amplitude of said high frequency oscillations in accordance with the nature of the wave selected from the low frequency signaling oscillations, a modulator for modulating low frequency signaling oscillations'and the high frequency oscillations controlled as to their amplitude, and means for transmitting the products of modulation. I

20. In a signaling system, the combination wit-h a source of low frequency signals, of a source of carrier frequency, means for increasing or decreasing the amplitude of the carrier frequency as the amplitude of the low frequency signals increases or decreases, respectively, means for superimposing the low frequency signals on the carrier frequency thus changed as to its amplitude, and means for transmitting the carrier frequency with the low frequency signals superimposed thereom i 21. In a signaling system, the combination with a source of low frequency signals, of a source of carrier frequency, means for means comprising a rectifier, a filter connected with the source of low frequency signals so arranged that the amplitude of the current passing said rectifier and said filter varies with the amplitude of the low frequency signals, and also comprising means for controlling the am litude of the carrier frequency by the amp itude of the current passing the said rectifier and the said filter,- and a modulator for sufperimposin the low frequency signals on t e carrier frequency controlled as to its amplitude in accordance 16 with the low fre uency signals.

In testimony w ereof, we have signed our names to this specification this 231' day of February, 1926.

- OTTO B. BLAGKWE'LL.

HERMAN A. AFFEL. 

